I think each animation gets exported separately.
So the image size (and the pivot) stays the same within one animation but not across different animations.
Maybe it can be fixed in a future version.
For now, you make a transparent box image, and place it behind your character at all times.
This will force Spine to export the images in the size of the transparent image as long as none of the skeleton images go beyond it.
In Unity, make sure you allow Unity to pack it tightly or in rects so it can trim out the transparent areas, to minimize fillrate effects and VRAM usage.
